1. Field of the Invention
This invention relates, in general, to a process for fabricating integrated circuit devices in which a layer of nitrided oxide material is formed and, in particular, to a method of forming nitrided oxide using a hot wall single wafer furnace.
2. Background of the Technology
Semiconductor devices such as MOS (metal-oxide-semiconductor) devices are typically formed on a substrate such as a silicon wafer. Typically, one or more films of an insulating material such as silicon dioxide are formed on the substrate over which is formed a gate electrode. The insulating film formed between the gate electrode and the silicon substrate is referred to as the gate oxide or gate dielectric. A widely employed type of MOS device is a complementary metal-oxide-semiconductor device, or CMOS device.
Boron doping of the gate electrodes of MOS devices (e.g., p+ gates) has been used to improve device performance by reducing short-channel effects and lowering threshold voltages. Typically, boron is implanted into the poly-Si gate at sufficiently high concentrations to ensure adequate conductance of the poly-Si gate. With the continued push for smaller and smaller MOS device dimensions, however, higher active dopant concentrations are required. When boron is used as the dopant for p+ gates, boron atoms in the gate layer can diffuse into the gate dielectric during downstream processing. Boron, which is a relatively small atom, has a very high diffusion coefficient in both silicon and silicon dioxide at temperatures encountered during processing. Further, it is necessary to activate the boron dopant after implantation with a high-temperature anneal which is typically conducted at temperatures in the range of 950–1050° C. During this high-temperature anneal, boron diffusion can be exacerbated.
Boron penetration into and through the gate dielectric can also have significant effects on device characteristics. First, boron penetration through the gate dielectric and into the channel can influence device performance. Boron diffusion into the channel, for example, can result in a shift in the threshold voltage of the device and can even result in charge-induced damage and breakdown during device operation. Also, as boron penetrates into the gate dielectric layer, the capacitance-voltage (C-V) or flat-band voltage of the device can shift which can degrade device performance. The presence of boron in the gate oxide film can also degrade the quality of the gate oxide film.
The reduction of boron penetration is particularly important in light of the decreasing dielectric layer thicknesses of modern MOS devices. It is known to incorporate nitrogen into an oxide film to retard the effects of boron penetration Nitrogen is believed to block boron diffusion by forming B-N complexes.
The amount of nitrogen incorporated into the gate oxide generally determines the effectiveness of the oxide layer in blocking boron diffusion. The amount of nitrogen doping required in a particular application, however, is dictated in part by the thermal cycles to which the device is subjected after deposition and doping of the gate electrode. Typical amounts of nitrogen required for adequate levels of boron diffusion blocking are in the range of 1–3 at. %.
Nitrogen has been incorporated into SiO2 using various methods. These methods include thermal oxidation followed by annealing in a nitrogen containing environment (thermal nitridation) and various deposition techniques such as physical vapor deposition (PVD) and chemical vapor deposition (CVD).
Various nitrogen containing gases have been employed for thermal nitridation and oxy-nitride deposition, including N2, NH3, NO and N2O. See, for example, U.S. Pat. Nos. 5,403,786; 5,521,127; 5,629,991; and 5,880,040. See also Gusev et al., “Growth and Characterization of Ultrathin Nitrided Silicon Oxide Films”, in IBM J. Res. Develop., Vol. 43, No. 3, pp. 265–286 (1999); Hook et al., “Nitrided Gate Oxides for 3.3-V Logic Application: Reliability and Device Design Considerations”, in IBM J. Res. Develop., Vol. 43, No. 3, pp. 393–406 (1999); and Buchanan, “Scaling the Gate Dielectric: Materials, Integration and Reliability”, in IBM J. Res. Develop., Vol. 43, No. 3, pp. 245–264 (1999). Evans et al. disclose a high pressure (15–25 atm.) process for oxynitride gate formation using nitric oxide gas. See Evans et al., “High Performance CMOS Devices with 20 Å Engineered Oxynitride Gate Dielectrics”, Paper Presented at Semicon Korea Technical Symposium, (February 2000). Kuehne et al. disclose rapid thermal nitridation of gate oxide layers using nitric oxide gas. Kuehne et al., Papers Presented at the Materials Research Society Spring 1997 Meeting, Rapid Thermal Processing, Kyoto (Spring 1997).
Current methods of nitriding gate dielectric layers involve annealing wafers in nitrous oxide gas at temperatures of about 900° C. Annealing is typically conducted in a batch process involving 100–160 wafers in each run. Conventional nitridation methods, however, can result in a relatively low concentration of nitrogen in the films. For example, oxide films that were either grown or annealed in N2O typically have total integrated nitrogen concentrations of less than 1 at. %. These relatively low concentrations of nitrogen are usually insufficient to reduce the effects of boron penetration from a p+ poly-Si gate into and through the gate dielectric layer. In order to incorporate sufficient amounts of nitrogen in the gate oxide layer, annealing has typically been conducted at relatively high temperatures (e.g., 900° C. and greater) and/or for relatively long times.
Current nitridation methods also tend to result in appreciable boron concentrations in the silicon channel layer at significant distances from the gate oxide/channel interface. The depth profile of the boron concentration should be such that the boron content as a function of distance from the gate oxide/channel interface drops rapidly on the channel side of the interface.
There still exists a need for a nitridation process which can rapidly incorporate sufficient amounts of nitrogen into the gate oxide layer of a MOS device. There also exists a need to form nitrided gate oxide layers having high concentrations of nitrogen in the gate oxide layer at the gate oxide/channel interface such that the concentration of boron dopant in the channel layer after activation of the dopant drops sharply as a function of the distance from the gate oxide/channel interface.